Waste to Energy - Sewage Gas

Sewage flows from our houses into the main sewer, from where it is pumped or flows by gravity to a sewage treatment plant which is usually sited at a low level.
Primary, secondary and tertiary treatment follows purifying the water and solids.
Methane gas is produced and used for power generation.

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Sewage treatment

Introduction

Sewage and wastewater from our homes is collected in the main sewage pipe which can be constructed from concrete, vitrified clay, or thermo-plastic and could be anything from 500 mm to 3 M in diameter. This pipe usually runs under the middle of the street downwards towards the sewage pumping station or the sewage treatment works, with numerous vertical pipes connecting it to surface manholes. On the way it collects rainwater from hard standings and from the roads; water containing hydrocarbon derivatives, tar, grit salt, and sand. This effluent flows by gravity to pump stations onto the treatment works, with the pipes getting larger in diameter on the way.

Once the waste matter reaches the treatment plant, it is pre-treated by passing through a number of screens and grit filters removing paper, cloth, grit and soil etc. which is usually incinerated or sent to landfill.

The remaining effluent can then be subjected to up to three treatments; Primary, Secondary and Tertiary. The end results are water being rid of impurities and able to be safely discharged, dried solids which can be recycled and, a gas produced which can drive gas turbines producing electrical power.

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Pre-treatment Stage

The effluent which reaches the treatment plant contains, as well as sewage, a range of materials which if not removed could damage pumps and clog filters. These materials are therefore removed in the pre-treatment phase being disposed off by incineration or landfill.

A system of screens usually constructed in a frame of vertical bars which have automatic scrapers to remove the debris. This is usually sent to landfill but can be incinerated the ashes being sent to landfill.

Sand, grit and stones are also removed by a system of course and fine screens in grit chambers and this can be washed and recycled or sent to landfill.

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Primary Treatment Stage

Once these materials have been removed the effluent is pumped by screw pump into sedimentation tanks where the 75% of suspended solids are separated from the water and, in the process, the biochemical oxygen demand and fecal coliform constituency in the water are halved.

The sedimentation causes sludge to build up at the bottom of the tank and this is pumped to a digester vessel. This is a pressure vessel in which the sludge is reduced in anaerobic conditions by micro-organisms to CO2 and methane gas. (More about this later.)

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Secondary Treatment Phase

The effluent is now passed into the aeration tanks where again micro-organisms break down the organic materials and nutrients in combination with compressed air which is constantly blown through the mixture. From here it is further clarified, with any remaining sludge being sent to the digesters and the wastewater going for tertiary treatment.

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Tertiary Treatment Phase

Before the water can be discharged, it must be purified to remove the nitrogen and phosphorus.

Nitrogen is removed by a process called de-nitrification which, when completed, allows the nitrogen to be released to the atmosphere.

The phosphorus is removed from the water by chemical precipitation or biologically. The sludge is rich in phosphorus can be dried and sold as fertiliser.

The removal of the nitrogen and phosphorus is important as they both can cause algae bloom, which in turn starves water of oxygen killing the indigenous biota.

Finally before discharge the water is disinfected using a variety of methods. The purpose of this final water treatment is to remove harmful microbes which still lurk in the wastewater after previous treatments.

The methods of disinfection are; chlorine dosing, UV light, and ozone treatment, but the most popular and the cheapest method is chlorination.